Technical Abstract:
This study was undertaken to determine the prevalence of Salmonella in the subiliac lymph nodes (LN) of cattle. Lymph node samples were obtained from carcasses of cull and feedlot cattle at commercial packing plants. Lymph nodes were trimmed of all fat, surface sterilized by submersion in boiling water for 3 seconds, placed in individual whirl-pak bags and then pulverized with a rubber mallet. Samples were enriched with tryptic soy broth and enrichments were subjected to immunomagnetic separation (IMS). IMS beads were transferred to Rappaport-Vasiliadis broth for secondary enrichment and Salmonella present were detected by culture on brilliant green sulfa and xylose lysine desoxycholate agars. Presumptive Salmonella isolates were confirmed by detection of invA by PCR, serotyped and their susceptibility to a panel of 15 antimicrobials determined. Between the months of September 2010 and July 2011, the mean prevalence of Salmonella in cattle subiliac LN (n = 2,564) was 5.3% (95% CI 1.3 - 8.8%). A seasonal and regional variation was observed with a Salmonella prevalence of 15.3% (95% CI 4.6 – 25.97) in the fall season from LN obtained from the Southern High Plains. Lymph nodes collected in the summer months demonstrated a seasonal increase in prevalence among samples originating in the Midwest and Southern High Plains. The majority of Salmonella isolated were serotypes Montevideo and Anatum. Enumeration of Salmonella from positive LN (n = 25) showed the geometric mean CFU/LN to be 2.9 x 10**3 (95% CI 8.5 x 10**2 – 9.7 x 10**3). The majority of Salmonella were pansusceptible (84.3%); however, tetracycline resistance and the MDR-AmpC phenotype were occasionally observed. These data demonstrate that Salmonella can readily be recovered from subiliac LN and consequently, beef trim containing LN may be a point-source for Salmonella entry into ground beef products. Moreover, harborage in LN protects Salmonella from the usual decontamination interventions used in packing plants. The public-health consequence of these findings needs to be investigated. Additionally, research is needed to better understand the opportunities to mitigate the risk of Salmonella harborage in LN of healthy cattle.